Why do I get a cube when I try the Fracture Modifier on a sphere?
Seems you already downloaded the Fracture Modifier Build. Here is a general introduction tutorial for it (its from 2014 but the basic principles work still in the same way.)

https://www.youtube.com/watch?v=9E0Nn_a6wCs

And the documentation site: 

https://wiki.blender.org/index.php/User:Scorpion81/Fracture_Documentation

Or you could just try to run the cell-fracture script in object mode. You tried to run it in edit mode where it obviously is not supposed to run and errors out then. By the way, the cell fracture script is completely independent of the Fracture Modifier.

When you use the Fracture Modifier (FM), you get a faster version of cell fracture with a lot more customization options. The smashing spheres could be 2 Objects, with a FM each. Just hit execute Fracture on both and Alt-A and the simulation starts (the spheres will fall down). Then you can animate the spheres, set them to Animated and Triggered, and make each sphere a Trigger, too. Optionally you could also use constraints within the FM. Then, when the spheres collide, they will break apart as well.